(1) Plasma membrane vesicles were prepared from tumor cells which are good target cells for cytotoxic T lymphocytes (CTL); by comparing their interaction with CTL before and after fusion of the vesicle membrane with the membrane of a tumor cell not recognized by the CTL, it was concluded that insertion of the H-2 antigen recognized into a living cell membrane greatly enhances recognition by CTL. (2) H-2 antigens on lymphoid cells were studied in binding experiments with labeled monoclonal anti-H-2. One example of a single H-2 antigen having two distinct determinants was found. (3) Spleen cells modified by several haptenic -SH reagents were found to stimulate the production of H-2 restricted hapten-specific CTL after in vitro culture with syngeneic spleen cells. Comparison with previously developed hapten modified CTL recognition systems shows the new system to be more H-2 restricted; the recognized hapten is on an -SH group not part of H-2. Epidermal cells modified with these reagents could also stimulate such CTL.